MPP sessions reuse stale channels after channel-not-found

Summary

MPP session auto-management can keep using a channel after the service rejects it with session/channel-not-found.

Moved from tempoxyz/accounts#520 because the stale channel cache and retry behavior live in MPPX client session management, not the Accounts SDK.

Expected

• If a session open fails verification, the client should not keep that channel as reusable.
• If a voucher request returns 410 Channel Not Found, the client should evict the cached channel and reopen on the next request.
• Apps should not need a hard refresh to clear the in-memory session channel map.

Actual

• The session client can cache a channel after creating an open payload.
• A later request can skip open and ask for a voucher against that channel.
• The service can return 410 with https://paymentauth.org/problems/session/channel-not-found, leaving the app stuck until refresh.

Product impact

The user signs a voucher, pays attention to wallet prompts, and then nothing happens. This makes paid agent flows look unreliable and makes it hard for apps to recover without exposing low-level session state.

Accounts context

Accounts wires MPPX through src/core/Provider.ts via mppx_tempo({ getClient }). The recovery behavior should be fixed in the MPPX session client; Accounts can add integration coverage once the upstream client evicts stale channels.

Start with src/core/Provider.ts and the mppx_tempo({ getClient }) wiring, then trace into the MPPX session client’s channel cache and voucher request flow. Done means failed verification does not preserve a reusable channel, and a 410 session/channel-not-found evicts the cached channel so the next request reopens the session.